The present invention relates to plain bearings, in particular, oil-lubricated bearings where very low friction, particularly static (breakaway) friction is essential, such as for automotive shock absorbers of the McPherson strut type.
Because of the requirement for very low friction, in these situations, in general only bearing materials with a very high proportion of Polytetrafluroethylene (PTFE) in the sliding surface are suitable since no materail other than PTFE appears to be able to achieve the low friction levels required. However, PTFE alone is very soft and weak and has an unacceptably high wear rate, thus harder materials must be added to the PTFE bearing surface to reduce wear and give an acceptable bearing life.
Bearings currently used in these applications consist of a steel backing having a porous bronze sinter layer and a bearing layer comprising filled PTFE. The bearing layer fills the intersticies of the sinter and forms a thin layer (approx. 25 microns) above the sinter. Some known compositions consist of PTFE filled with either 20% by volume of lead, molybdenum disulphide, graphite, bronze, etc.
These conventional materials tend to suffer erosion effects when used under severe conditions in which cavitaion occurs in the lubricating oil. Such conditions can occur at high conterface velocities in such applications as gear pumps and aviation fuel pumps, and automotive shock absorbers where the rod speed relative to the bearing surface moves at abnormally high speeds (greater than 3 m/sec.). This form of cavitation erosion results in the removal of the filled PTFE phase from the sinter by the actioin of the lubricating oil alone, without rubbing wear.